A hybrid vehicle and an electric vehicle, in which a motor is mounted as a drive power source of a vehicle, are proposed to meet social demands for low fuel consumption and low exhaust emission. As a power control system for such a vehicle, JP 2007-295699A (US 2009/0108674A1) discloses one exemplary system. In this system, an inverter, a bidirectional DC-DC converter, a capacitor and the like are connected to a battery (main charge storage device) such as a secondary battery through a main relay, which has positive-side and negative-side switches. The inverter is connected to a motor and the DC-DC converter is connected to an accessory battery (charge storage device for accessory equipment).
According to this system, the DC-DC converter is driven with the positive-side and the negative-side switches of the main relay being tuned off to charge the capacitor until a voltage VL across both terminals of the capacitor reaches a predetermined voltage Vth1. Then the voltage across the terminals of the capacitor is compared with a reference voltage Vth2 with the positive-side switch and the negative side switch of the main relay being turned on and off. Thus it is checked whether the negative-side switch of the main relay is short-circuited by a relay switch fixedly welded to a terminal.
In some power control apparatuses for a plug-in hybrid vehicle and an electric vehicle, which is chargeable by an external power source provided outside the vehicle, a charger relay is provided in a power cable connecting a battery and a charger connectable to the external power source.